In a general swash plate type piston pump, a rotating shaft and a fixed cylinder block are provided in a casing of the swash plate type piston pump, and front end portions of a plurality of pistons extending substantially in parallel with the rotating shaft are inserted into the cylinder block (see Japanese Laid-Open Patent Application Publication 11-50951 for example). Rear end portions of the pistons are introduced to a front surface of a swash plate inclined with respect to the rotating shaft. The pistons reciprocate by the rotation of the cylinder block to suck/discharge hydraulic oil. A circular- convex portion is formed on a rear surface of the swash plate, and is supported by a circular- recess of a swash plate support. Then, lubricating oil is supplied to a supporting surface of the swash plate support, and the swash plate is caused to tilt with respect to the rotating shaft. Thus, the stroke of the piston changes to adjust the amount of hydraulic oil discharged. At this time, the increase in a tilt angle of the swash plate increases the stroke of the piston, thereby increasing the amount of hydraulic oil discharged, whereas the decrease in the tilt angle of the swash plate decreases the stroke of the piston, thereby decreasing the amount of hydraulic oil discharged.
In the foregoing swash plate type piston pump, since a reaction force applied by the hydraulic oil to the pistons when the pistons move back into the cylinder block to discharge the hydraulic oil acts on the swash plate, a surface pressure between the swash plate and the swash plate support becomes very high. Therefore, a lubricating oil film at an interface between the swash plate and the swash plate support tends to run out. On this account, friction surfaces of the swash plate and the swash plate support require seizing resistance and abrasion resistance. Conventionally, the seizing resistance and the abrasion resistance are given to the swash plate support, made of cast iron, by carrying out a surface hardening heat treatment, such as a gas nitrocarburizing, with respect to the swash plate support. Moreover, in the case of a comparatively large pump, the seizing resistance and the abrasion resistance may be given to the swash plate support by carrying out a copper alloy lining with respect to the supporting surface of the swash plate support.
In a piston pump, a rotational power transferred to the rotating shaft is an input, and the hydraulic oil discharged by the piston is an output. In contrast, in a piston motor, the inflow of pressure oil is an input, and the rotational power of the rotating shaft is an output. To be specific, although how to use the piston pump and how to use the piston motor are different from each other, the piston pump and the piston motor are basically the same as each other in configuration. Therefore, such configuration is referred to as a piston pump motor in the present description.